1. Related Applications
This application is related to U.S. Ser. No. 270,173 filed June 3, 1981, and U.S. Ser. No. 06/360,746 filed Mar. 22, 1982, both on inventions of Koepp which are assigned to Fairchild Camera and Instrument Corporation, the assignee of this application, and both of which are hereby incorporated by reference.
2. Field of the Invention
This invention relates to electronic circuits for use in telephone apparatus.
3. Description of the Prior Art
A schematic diagram of a standard 500 type telephone set, a telephone industry standard, is shown in the March 1979 application noted entitled "S2559 Digital Tone Generator" published by American Microsystems, Inc., which is herein incorporated by reference. Of importance, the "encapsulated circuit" depicted within the dashed lines of the schematic of the 500 type telephone set described in this application note functions to separate the signals on the two wire phone line for use in the receiver transducer and the transmitter transducer. Furthermore, the "encapsulated circuit" is used to provide regulation, wherein the amplitude of the received signal and the amplitude of the transmitted signal are relatively constant in the telephone set regardless of the set's distance from the central switching office. The "encapsulated circuit" also controls the amplitude of the signal transmitted by a telephone set which appears in the receiver of this same telephone set, commonly called sidetone. It has been found that some sidetone is beneficial in maintaining a natural conversation, while an excessive sidetone amplitude will detract from the conversation.
Of importance, the "encapsulated circuit" of the standard 500 type telephone set employs a plurality of discreet components including, in the 500 type telephone set, four discreet capcitors, three discreet resistors, two discreet varistors, two discreet iron core inductors, and one iron core transformer. Prior art telephone sets other than the 500 type are quite similar, employing a plurality of discreet components including iron core inductors and an iron core transformer. The use of such components results in prior art telephones being rather expensive, as well as rather bulky and heavy.
One approach to eliminating the need for the "encapsulated circuit" of the standard prior art telephones is presented by Erratico, et al in an article entitled "The First Electronic Speech Circuit Opens the Way to the Fully-Electronic Telephone", IEEE Transactions on Consumer Electronics, Vol. CE-26, February, 1980 pp. 73-80. Erratico et al utilize one approach to substitute an integrated circuit for the prior art "encapsulated circuit" comprised of discreet components. Another approach is disclosed by the two above-mentioned related patent applications.